Recently, demand for high-strength steel sheets for improving vehicle fuel economy to meet with environmental protection regulations has rapidly increased. In accordance with the strengthening of automotive steel sheets, wear and fracturing may occur therein during press forming, and thus it may be difficult to form products having complex shapes. Therefore, in order to resolve such limitations, the production of products through a hot pressing process in which steel sheets are pressed in a heated state has been markedly increased.
Steel sheets for hot pressing are generally subjected to hot press forming in a temperature range of 850° C. to 930° C., and thus are likely to be oxidized and coated with oxide scale during heating. Therefore, an additional process such as shot blasting may be necessary to remove such scale after product formation, and thus, corrosion resistance of such products may be inferior to that of plated materials.
Therefore, in methods proposed to address such limitations, such as that disclosed in U.S. Pat. No. 6,296,805, a steel sheet is plated with an aluminum (Al)-containing material resistant to the heating furnace environment so as to suppress surface oxidation of the steel sheet and increase the corrosion resistance of the steel sheet by the formation of a passive Al film. Products using such methods have been developed and commercialized.
However, although such Al-plated materials have good heat resistance at high temperatures, corrosion resistance thereof is inferior to that of materials plated with zinc (Zn) through a sacrificial anode method, and manufacturing costs may also increase.
On the other hand, high-temperature heat resistance of zinc (Zn) is significantly inferior to that of Al. Therefore, at a high temperature of 850° C. to 930° C., a steel sheet plated with Zn by a typical plating method is subjected to alloying of Zn and oxidation, which make the zinc plating layer unstable and reduce the Zn content of the zinc plating layer to less than 20%. Therefore, the zinc plated steel sheet is deteriorated in terms of corrosion resistance.
In addition, if the zinc plated steel sheet is heated and hot-pressed, an alloy phase of the zinc plating layer having a high Zn content may become liquid and make contact with the steel sheet, rendering the surface of the steel sheet brittle (through liquid metal embrittlement, LME) and thus causing damage such as cracking.